|Small Size and Multiple Colors|
This time I'm talking about a directional lav; the Countryman B2D Cardioid Lav. That's right, a cardioid lav that comes in five different colors. Most lavs are omnis, and as you may know, even omnis are somewhat directional at high frequencies.
You can see that the B2D is a pretty small mic. Not quite as small as a B6, but a lot smaller than you'd expect. The diameter of the diaphragm is inversely proportional to the selfnoise of any mic. That means the smaller the diaphragm, the noisier the mic. If your ambient noise is high enough, it'll mask the hiss of the selfnoise, but if you're in a really quiet environment and don't have something like music to mask the hiss, you'll hear it.
You can use some of the many noise reduction softwares to reduce the hiss. I like iZotope's RX3 Advanced. You may find you get acceptable results with the less expensive non-advanced version. With any of these, you need to have a keen ear and know when to stop because at some point you can hear the effect of the noise reduction. The audio begins to sound "underwater" or unnatural.
Here's a short video that demonstrates the difference between a Countryman EMW omnidirectional lav and the cardioid B2D. Hear for yourself!